In making a coated paper, a method of coating a paper web, which is continuously moving on a paper machine, on the downstream side of the drying part of the paper machine has been widely used because it makes the production process simpler and more efficient as compared with off-machine coating.
In contrast with the off-machine coating in which a coating composition is applied to a paper web at an ambient temperature, in on-machine coating the coating composition permeates into the paper web rapidly because the surface temperature of the paper web to be coated with the coating composition is ralatively high. Therefore, unless the amount of coating is considerably increased, the coating cannot correct the unevenness of the paper web itself, and the coated surface on the paper web is uneven as well.
A coated paper having an uneven coated surface is not suitable for use as a printing paper because the ink receptivity and water absorption thereof are uneven. If such a coated paper is used as an undercoated base paper for a thermosensitive paper, the thermosensitive material will cause uneven color development. The thermosensitive paper comprises an undercoated base paper coated on its surface with a thermosensitive coating layer, the weight of said coating usually being as small as about 5 g/m.sup.2. Therefore, the surface evenness of the undercoated base paper is very important to obtain a good recording image.
There are various means for preventing the coating composition from rapidly permeating into a paper web. One of them is to make the size stronger or raise the density of the paper web. However, the degree of permeation of the coating composition into the paper web depends not only upon the size properties and density of the paper web but also upon the viscosity of the coating composition and the surface temperature of the paper web at the time of coating. However, in the conventional on-machine coating, the interrelations between these factors have not been made clear.